In the U.S., breast cancer is the leading cause of cancer and second leading cause of cancer-death in women, while ovarian cancer is the most fatal gynecologic cancer. Breast and ovarian cancer have been found to co- occur. Both female cancers are associated with mutations in BRCA1 or BRCA2; however mutations in these genes account for only a small percent of all breast and ovarian cancers and less than half of all familial cancers. Thus, identifying other factors and pathways that are important to women at greatest risk of developing one or both cancers is crucial. Epigenetic modifications, such as DNA methylation, refer to heritable and modifiable markers that regulate gene expression without changing the underlying DNA sequence. Aberrant DNA methylation in specific genes can activate or silence genes critical to tumor development and progression; methylation of key genes has been observed in both breast and ovarian cancer. However, little research has been conducted to examine DNA methylation in high risk women from cancer families, many of whom do not carry a mutation in BRCA1 or BRCA2. Using the unique resource of the Breast Cancer Family Registry (BCFR), an international, multi-ethnic registry of breast and ovarian cancer families, I propose to examine the following aims: 1) To examine the association between hormonal, reproductive and anthropometric risk factors (OC use, parity, breastfeeding, and height) and breast and ovarian cancer co- occurrence using already existing retrospective data on 6,032 high-risk women; 2) To compare gene-specific DNA methylation in 8 genes (BRCA1, BRCA2, ER, PR, APC, RASSF1A, MLH1, P16) in tumor and adjacent non-tumor tissue from women with ovarian cancer (n=24 pairs), women with breast cancer (n=24 pairs), and women with breast and ovarian cancer (n=24 pairs); and 3) To compare gene-specific DNA methylation in genes found to be important in Aim 2, in the plasma of women who prospectively developed ovarian cancer compared to controls matched on age and date of diagnosis and race/ethnicity (n=100 pairs). The proposed training plan leverages my epidemiologic training by adding laboratory and biostatistical training relevant to molecular epidemiology and specifically to large scale DNA methylation analyses. The successful completion of these aims will provide critical data missing from the literature on the role of epigenetics and epidemiologic risk factors in the co-occurrence of breast and ovarian cancer. By examining complementary strategies using tissue and plasma-based markers, we will also robustly test whether markers in plasma can be used as potential screening for ovarian cancer risk in high-risk women. Completion of this proposed research project and training plan will provide essential data for future research projects that I will be able to use to further develop my career as an independent researcher.